Chapter 14 Vibrations and Waves

Hooke’s Law F s = - k x F s is the spring force k is the spring constant It is a measure of the stiffness of the spring A large k indicates a stiff spring and a small k indicates a soft spring x is the displacement of the object from its equilibrium position x = 0 at the equilibrium position The negative sign indicates that the force is always directed opposite to the displacement

Hooke’s Law Force The force always acts toward the equilibrium position It is called the restoring force The direction of the restoring force is such that the object is being either pushed or pulled toward the equilibrium position

Hooke’s Law Applied to a Spring – Mass System When x is positive (to the right), F is negative (to the left) When x = 0 (at equilibrium), F is 0 When x is negative (to the left), F is positive (to the right)

Motion of the Spring-Mass System Assume the object is initially pulled to a distance A and released from rest As the object moves toward the equilibrium position, F and a decrease, but v increases At x = 0, F and a are zero, but v is a maximum The object’s momentum causes it to overshoot the equilibrium position

Motion of the Spring-Mass System, cont The force and acceleration start to increase in the opposite direction and velocity decreases The motion momentarily comes to a stop at x = - A It then accelerates back toward the equilibrium position The motion continues indefinitely

Simple Harmonic Motion Motion that occurs when the force is proportional to the displacement and always directed toward the equilibrium position The motion of a pendulum is an example of Simple Harmonic Motion

Simple Harmonic Motion, cont. Not all periodic motion over the same path can be considered Simple Harmonic motion To be Simple Harmonic motion, the force needs to obey Hooke’s Law

Simple Pendulum The simple pendulum is an example of simple harmonic motion The force is the component of the weight tangent to the path of motion F t = - m g sin θ

Period of Simple Pendulum This shows that the period is independent of the amplitude The period depends on the length of the pendulum and the acceleration of gravity at the location of the pendulum

Wave Motion A wave is the motion of a disturbance Mechanical waves require Some source of disturbance A medium that can be disturbed Some physical connection between or mechanism though which adjacent portions of the medium influence each other All waves carry energy and momentum

Types of Waves – Traveling Waves Flip one end of a long rope that is under tension and fixed at one end The pulse travels to the right with a definite speed A disturbance of this type is called a traveling wave

Types of Waves – Transverse In a transverse wave, each element that is disturbed moves in a direction perpendicular to the wave motion

Types of Waves – Longitudinal In a longitudinal wave, the elements of the medium undergo displacements parallel to the motion of the wave A longitudinal wave is also called a compression wave

Waveform – A Picture of a Wave The brown curve is a “snapshot” of the wave at some instant in time The blue curve is later in time The high points are crests of the wave The low points are troughs of the wave

Longitudinal Wave Represented as a Sine Curve A longitudinal wave can also be represented as a sine curve Compressions correspond to crests and stretches correspond to troughs Also called density waves or pressure waves

Description of a Wave A steady stream of pulses on a very long string produces a continuous wave The blade oscillates in simple harmonic motion Each small segment of the string, such as P, oscillates with simple harmonic motion

Amplitude Amplitude, A The amplitude is the maximum position of the object relative to the equilibrium position In the absence of friction, an object in simple harmonic motion will oscillate between the positions x = ±A

Amplitude and Wavelength Amplitude is the maximum displacement of string above the equilibrium position Wavelength, λ, is the distance between two successive points that behave identically

Period and Frequency The period, T, is the time that it takes for the object to complete one complete cycle of motion From x = A to x = - A and back to x = A The frequency, ƒ, is the number of complete cycles or vibrations per unit time ƒ = 1 / T Frequency is the reciprocal of the period

Speed of a Wave v = ƒ λ Is derived from the basic speed equation of distance/time This is a general equation that can be applied to many types of waves

Interference of Waves Two traveling waves can meet and pass through each other without being destroyed or even altered Waves obey the Superposition Principle If two or more traveling waves are moving through a medium, the resulting wave is found by adding together the displacements of the individual waves point by point Actually only true for waves with small amplitudes

Constructive Interference Two waves, a and b, have the same frequency and amplitude Are in phase The combined wave, c, has the same frequency and a greater amplitude

Constructive Interference in a String Two pulses are traveling in opposite directions The net displacement when they overlap is the sum of the displacements of the pulses Note that the pulses are unchanged after the interference

Destructive Interference Two waves, a and b, have the same amplitude and frequency They are 180° out of phase When they combine, the waveforms cancel

Destructive Interference in a String Two pulses are traveling in opposite directions The net displacement when they overlap is decreased since the displacements of the pulses subtract Note that the pulses are unchanged after the interference

Reflection of Waves – Fixed End Whenever a traveling wave reaches a boundary, some or all of the wave is reflected When it is reflected from a fixed end, the wave is inverted The shape remains the same

Reflected Wave – Free End When a traveling wave reaches a boundary, all or part of it is reflected When reflected from a free end, the pulse is not inverted